This invention relates to a gas detection apparatus and a method for controlling a gas sensor.
In general, a polymer electrolyte fuel cell is provided in the form of a fuel cell stack comprised of a plurality of stacked single cells each having an anode (fuel pole), a cathode (oxygen pole), and a solid polymer electrolyte membrane sandwiched between the anode and the cathode. The anode is supplied with hydrogen as a fuel, and the cathode is supplied with air as an oxidant, so that hydrogen ions produced by catalytic reaction at the anode pass through the solid polymer electrolyte membrane to the cathode in which the hydrogen ions and the oxygen react electrochemically to generate electricity.
In the polymer electrolyte fuel cell as described above, there may be provided a protective device having a hydrogen sensor (gas detection apparatus) disposed in an exhaust system of a gas discharged from the cathode of the fuel cell, so that supply of fuel is cut off when the hydrogen sensor detects leakage of hydrogen from the anode to the cathode through the solid polymer electrolyte membrane (see JP 6-223850 A, for example).
Among various types of hydrogen sensors known in the art is a catalytic combustible gas sensor. The catalytic combustible gas sensor includes a gas sensing element made of a catalyst (e.g., platinum), and a temperature compensated element. Hydrogen catalytically burns at the gas sensing element when it comes in contact with the catalyst such as platinum; thus, the temperature of the gas sensing element rises relative to the temperature of the temperature compensated element. This generated difference in temperature between the gas sensing element and the temperature compensated element may be represented by a difference in electric resistance. The catalytic combustible gas sensor measures a concentration of hydrogen using such a difference in electric resistance between the gas sensing element and the temperature compensated element.
Exhaust gases discharged from the fuel cell may possibly contain traces of silicon derived from materials of a seal member or the like provided in the exhaust system or piping. Silicon that comes in contact with the catalyst of the gas sensing element may probably poison the catalyst, thus impairing the sensing accuracy of the gas sensing element. With this in view, we, applicants of the instant application, have previously proposed a method for diagnosing a deterioration of a gas sensor (gas detection apparatus), using a first gas sensing element for regular use and a second gas sensing element for use in determination of the deterioration of the first gas sensing element. The first gas sensing element is supplied with electric power continuously, while the second gas sensing element is supplied with electric power intermittently, i.e., normally not energized. This prior invention is disclosed in JP 2004-251862 A (related applications were also published under WO 03/096000 A1, CA 2 485 604 A1, EP 1 505 385 A1 and US 2005/0155405 A1), the disclosure of which is herein incorporated by reference in its entirety.
Typically, air to be supplied as an oxidant to the cathode of the fuel cell is humidified to maintain the ion conductivity of the solid polymer electrolyte membrane, and the exhaust gases discharged from the fuel cell have a moisture content generated by the reaction of hydrogen and oxygen. This may cause condensation on the gas sensing elements on occasions. Since the second gas sensing element of our prior invention described above is not energized in normal times, adhesion of water drops on this second gas sensing element would not only prolong the time required for starting the second gas sensing element but also make the second gas sensing element underperforming, i.e., lowering sensitivity thereof.
Thus, it would be desirable to provide a gas detection apparatus and a method for controlling a gas sensor, by which deterioration of the gas sensor can be detected, and in which the gas sensor can be swiftly started and durability of the gas sensor is enhanced. The present invention has been made against this backdrop.
Illustrative, non-limiting embodiments of the present invention overcome the above disadvantages and other disadvantages not described above. Also, the present invention is not required to overcome the disadvantages described above, and an illustrative, non-limiting embodiment of the present invention may not overcome any of the problems described above.